Reactor Theory (Reactor Operations)DOE-HDBK-1019/2-93REACTOR OPERATIONAs reactor power increases to a level above the level of the new energy demand, the temperatureof the moderator and fuel increases, adding negative reactivity and decreasing reactor powerlevel to near the new level required to maintain system temperature. Some slight oscillationsabove and below the new power level occur before steady state conditions are achieved. Thefinal result is that the average temperature of the reactor system is essentially the same as theinitial temperature, and the reactor is operating at the new higher required power level. Thesame inherent stability can be observed as the energy demand on the system is decreased.If the secondary system providing cooling to the reactor heat exchanger is operated as an opensystem with once-through cooling, the above discussion is not applicable. In these reactors, thetemperature of the reactor is proportional to the power level, and it is impossible for the reactorto be at a higher power level and the same temperature.PressureThe pressure applied to the reactor system can also affect reactor operation by causing changesin reactivity. The reactivity changes result from changes in the density of the moderator inresponse to the pressure changes. For example, as the system pressure rises, the moderatordensity increases and results in greater moderation, less neutron leakage, and therefore theinsertion of positive reactivity. A reduction in system pressure results in the addition of negativereactivity. Typically, in pressurized water reactors (PWR), the magnitude of this effect isconsiderably less than that of a change in temperature. In two-phase systems such as boilingwater reactors (BWR), however, the effects of pressure changes are more noticeable becausethere is a greater change in moderator density for a given change in system pressure.PowerLevelA change in reactor power level can result in a change in reactivity if the power level changeresults in a change in system temperature.The power level at which the reactor is producing enough energy to make up for the energy lostto ambient is commonly referred to as the point of adding heat. If a reactor is operating wellbelow the point of adding heat, then variations in power level produce no measurable variationsin temperature. At power levels above the point of adding heat, temperature varies with powerlevel, and the reactivity changes will follow the convention previously described for temperaturevariations.The inherent stability and power turning ability of a negative temperature coefficient areineffective below the point of adding heat. If a power excursion is initiated from a very lowpower level, power will continue to rise unchecked until the point of adding heat is reached, andthe subsequent temperature rise adds negative reactivity to slow, and turn, the rise of reactorpower. In this region, reactor safety is provided by automatic reactor shutdown systems andoperator action.Rev. 0NP-04Page 29
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